This invention relates to optical refractometers and in particular to an optical fiber refractometer in which light transmission through an immersed fiber indicates refractive index.
The index of refraction of a liquid may be determined by filling a prismatic cell with the liquid of interest and by transmitting a collimated beam of light through the filled cell. The light transmitted through the cell is measured and the deviation of the beam due to refraction is determined. This method requires cumbersome optical instrumentation, manual adjustments, calculations, and handling of the liquid. It also requires liquid volumes in excess of one cubic centimeter.
Thus, methods using fiber optics were developed to permit the measurement of the index of refraction of liquids with a smaller volume of liquid. A basic fiber optic device for measuring the index of refraction of a liquid is taught in Canadian Patent No. 1,184,780, issued to Jacques P. Legendre, entitled "Apparatus for Monitoring Refractive Index Changes in Fluids." In the apparatus taught by Legendre, an optical fiber was illuminated at one end while the other end was in contact with the fluid whose refractive index was to be measured. The optical energy reflected back from the interface between the end of the fiber and the liquid depended on the relative values of the refractive index of the liquid and the refractive index of the fiber. A signal was generated corresponding to the reflected energy and an electrical signal was generated corresponding to the energy provided by the optical source. From these measurements, the refractive index of the liquid was calculated.
Japanese Patent Application No. 59-97046, entitled "Device for Determination of Refractive Index of Fluid", disclosed a device wherein a curved light transmissible object was immersed in the liquid to be monitored. Light striking the walls of the light transmissible object due to the curvature of the object was partially reflected and partially refracted depending on the relative index of refraction of the light transmissible object and the liquid. The light travelled through the light transmissible object and was reflected from a reflecting section at the end of the object back to a photosensitive device. The amount of returning light was a measure of the refractive index of the liquid. However, such a device still required a relatively large volume of liquid since it was necessary to immerse the curved portion of the light transmissible object in the liquid.
U.S. Pat. No. 4,564,292, issued to Omet, disclosed a device requiring less liquid in order to make a measurement. In the device of Omet, a refractometer was provided on the tip of a measuring probe. The refractometer included a curved portion of a light conducting medium to permit measurement of a small amount of liquid by immersing the tip in the liquid to be measured.
A fiber optic refractometer using a tapered optical fiber is taught in "Novel Refractometer Using a Tapered Optical Fibre", by A. Kumar, in Electronics Letters, June 21, 1984, Volume 20, No. 13, page 534. The refractometer taught by Kumar is provided with an optical fiber adapted for immersion in the liquid being measured. One portion of the optical fiber had one diameter and a second portion of the optical fiber had a second diameter. A tapered region was provided between the two portions having different diameters. Light was transmitted from the larger diameter portion through the tapered region to the smaller diameter portion. The amount of light energy applied to the smaller fiber through the tapered region is related to the relationship between the indices of refraction of the optical fiber and the liquid in which the optical fiber is immersed. However, the optical fiber refractometer taught by Kumar was still limited in the range of indices it could measure from about n=1.33 to n=1.44.